Lithographic printing plate (after process) generally consists of ink-receptive areas (image areas) and ink-repelling areas (non-image areas); the image areas and the non-image areas are substantially on the same plane. During printing operation, ink is preferentially received in the image areas, not in the non-image areas, and then transferred, usually through a printing blanket, to the surface of a material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties (such as hydrophilic vs. oleophilic, and oleophobic vs. oleophilic). The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. Here, hardening means becoming insoluble in a certain developer. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. The exposed plate is usually developed with a liquid developer to bare the substrate in the non-hardened areas.
On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after exposure to develop with ink and/or fountain solution during the initial press operation and then to print out regular printed sheets. No separate development process before mounting on press is needed. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, and 6,242,156.
Conventionally, the plate is exposed with an actinic light (usually an ultraviolet light from a lamp) through a separate photomask film having predetermined image pattern that is placed between the light source and the plate. While capable of providing plate with superior lithographic quality, such a method is cumbersome and labor intensive.
Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser wavelength. This allows the elimination of the photomask film, reducing material, equipment and labor cost.
Among the laser imagable plates, infrared laser sensitive plates (also called thermosensitive plates or thermal plates) are the most attractive because they can be handled and processed under white light, at least for limited time. Various thermosensitive plates have been disclosed in the patent literature. Examples of thermosensitive plates are described in U.S. Pat. Nos. 4,054,094 and 5,379,698 (laser ablation plates), U.S. Pat. Nos. 5,705,309, 5,674,658, 5,677,106, 6,153,356, 6,232,038, and 4,997,745 (negative thermosensitive plates), U.S. Pat. Nos. 5,491,046 and 6,117,610 (both positive and negative thermosensitive plates, depending on the process), and U.S. Pat. Nos. 5,919,600 and 5,955,238 (thermosensitive positive waterless plate).
Despite the progress in conventional on-press developable plates and digital laser imagable plates, there is a desire for a on-press developable thermosensitive lithographic printing plate and method which utilize infrared laser or other infrared radiation, do not produce ablation debris, do not require a separate liquid development process, and are easy to use.